Radio direction finding



ct. 24, 1933.` Q SCHELLENG ,93,853

RADIO DIRECTION FINDING Filed Aug. 23, 1929 J SCHELLENG By/Myw/wa A TTUH/VEY massi PatentedV Oct. 1933 l 1,931,853 Y Y anoto plano-'HON `FiNDiNa Joimc.. 'sandalias/.tintura N. J., assignor to 'Beil Telephone Laboratories', Incorporated', Y lNew' York, N.` Y., a corporation of 'New York Applicata@ Augustas, 1929. serial No. 387,826

' l J claims, (01. 25o-11) ing, that is, to methods or means whereby a given f radio station may determine the direction of another radiostation. It especially relates to meth- This invention relates lto radio direction find1 ods or meansv whereby the direction of a given radio transmitting station maybe substantially determined Vby a determination lof the direction of incidence ofradio waves emanating from it on a receiving radio station. Y' 'Y Y An object of the invention is to insure Vgreater accuracy than by prior methods ormeans, in the determination'ofthe direction of incidence ,of a' radio 'wave von a receivingstation.`

A more particular object is to determine such direction of incidence without reference to the .o direction of the electricvector at thevincident sta` tion Aand without detriment to accuracy due to deviation ofthe direction offsuchV vector from the vertical orl from its initial direction, as attends thel operation of similar direction finding systems;

A feature of thev invention'by reason principally ofwhich these and other objectsare` achieved relatestotheu'se of the energy (Poynt- `ing) vector-'as the-referencequantity inthe di-vrection determination, this vector, by definition,

indicating the directionjof wave lpropagation in 'the ether.

VIn general there are three'representative'char# `acteristics of an ether wave which may be made use of in determining the direction ofincidence of such a wave on a receiving station, namely the electric, magnetic and energy characteristics.

These characteristics are interdependent as shown 1 by classical treatmentsfofelectromagnetic wave propagation. These quantities may convenient-f ly be represented graphically by vectors.`

By itself, the. electric Vvector at a point is not useful in determining the direction of propagation of a wave in the earthsplane Where, asin. the usual case, the electricvector'is substantially vertical as results from the use of a vertical .trans- Vvector and the direction of propagation vare nor-o mal to each other, this fact in the practical'case. above noted would only denne the'earths plane" mitting antenna.V Although'it is known that vsuch and would indicate nothing Yof the (direction of propagation inpthat plane.AV There is an important difference however when the magnetic vector Yis correspondingly considered, vresulting from ythe known fact that the electric and magnetic,vec tors and direction oi propagation are mutually j right-angularly related and ,stillV assuming `the above-mentioned case where the electricv vector is vertical.. `For this case the resultant determination-fist` deinitive of the actual' direction 1 of propagation rather than merely of the plane in which the direction vector exists. When the elec- `Vtical vector of the incident wave is strictly vertical the direction of propagation along a given radius must be' in thefearths plane and specifically in the direction of the horizontal vector.

Otherwise it must have a component in the earths plane whichv wouldv be correspondingly determined by a determination of the" direction of: thev horizontal component of the magnetic-Y vector. This has therefore been the customary method of vdirection nding. y f

The difculty in these prior methods is to obtain anaccurate determination of the direction of thisI 'horizontal component of the Amagnetic vector.

For example, it is well-known that with the conventional loopl rotatable about a vertical axis a null reading is not Vnecessarily obtainable for any position of the loop. Thus, for example, as will be explained in detail later, under circumstances` where-,av rotation of the plane `of polarization is combined with al downwardl component of' the j direction of propagation, a null position will-not be found with'the plane of thevertical loopperv Ypendicular tothe direction of arrival in the hori-VV zontal plane.-V This. follows from the resultant dissymmetry between the electromotivev forces generated in the upper and lower sides' of the loop.Y Furthermore by reason of this kdissyrnmetry, a minimum reading does not indicate that -the plane of the loop is parallel to the direction of the magnetic rvector or therefore normal to the'direction of propagation.` The method or system ofthe invention avoids this error.

VA mathematical expression for thecomponent L of the energy vector in any givenv direction in the earths plane is readily derived, this direction ydetermining one axis in terms of the corresponding electric and magnetic vector components with respect to the other two vcoordinate axes.

.In the application of the mathematical expressions to the present invention this axis is'chosen sonas to have aA definite relation to the direction of propagation of the-.wavevThe expression is the difference of two products; of electric and" Y, magnetic` components.

. The measurements of the electric and magnetic components along the two axes required by the expression are thereforeeasily accomplishedrby open andv loop antenna. The products of the two may be determined bya wattmeter type of device. Y The receiving structure consists simply ofY a linear antenna and a loop antenna rigidly connected together so. as`to bef'coplanar and capable of rotaf.

Vtion,`the'wholerotatable about both afvertical and a horizontal axis. The technique of operaawave reflected from the earth, in addition to the normal one, is taken into account.

Other objects and features of the inventionv will appear from the following detailed description taken in connection with the accompanying drawing; in which Fig. 1 discloses an antenna whereby the combination of open and loop antennae of the inventionmay, as a unit, beorientated about two right-angularly related axes; and

A Fig. 2 discloses a system as a Whole embodying .the invention, including certain portions ofthe disclosure of Fig. 1 diagrammatically.

. v. Preliminary toa description of the mechanism and procedure by ,which the invention is practiced,

ananalysis of "the theoretical principles upon which the invention is basedjand whichis required in order to. intelligently practice the invention, will be undertaken.

y The invention involvesI a meansrfor determining the direction of the-Poynting vector, that is,

the vector representing the direction of iiow of energy in the ether. While the other two repre- -sentative vectors. that are useful in defining a wave, namely, theV electric and magnetic eld vectors, may be used in the determination of direction of propagation, since theirk directions have a relation thereto, itis well known, and it `will beshown later, that errors are introduced in their usage under certain circumstances. rThe determination of the direction of thePoynting vector .incontrovertably establishesthe direction (of propagation of the wave and therefore its direction of arrival at a receiving station independently.

of the conditions which -causeV errors in prior alternative methods. Y'

- In vector notation, the electric and magnetic be vexpressed as Y I portions of the loop when the looprisin a plane fields of an ether wave may.

follows;

These equations represent A.thetotal field due to the incidentand reflected waves. In them the quantities E and H respectively represent, asis andthe coefcents i, y and 7c being numerical constants.

The Poynting'vector, since it is a measure of v the total wave energy, is measured by the vector product ofthe above equations.-This product may be mdlcateq m deermulravnt formt as rfol'f a` Way as to determine the direction in which the If the yl axisis taken perpendicular to the surface of the earth, then, irom'the next abovejjwith. The open antenna gives a response, for a ',igven'requntly numefiauy proportional to the equation, the net iiow of` energy vertically is mounting means hence a` position This component of the energy flow discloses nothing with regard to the direction of arrival of thewave at a given point in the equatorial plane and hence it need not be further considered. That is, direction finding apparatus should not be affected by it. Y

Correspondingly, the flow of energy in the a direction is given by EzHy-EyHr, and the ilow of energy in the x direction by EyHrExHy.

If we consider our system of axes to rotate about the y axis as we rotate the direction i'nd- Vin the art that although apparatus vwhich indicates that theenergy oW in the a direction Vis zero may be used to indicate the true direction of propagation, the direction cannot be accurately indicated under all circumstances by the luseof a loop alone, such ashas beenfcustoma'rily used to determine the direction `of the horizontal component of the magnetic eld.

In proof of the last statement above, consider the `practicalcase Where the wave front has ra tilt from the vertical as maybe due to propagation upwardly against, and reflection from, the ionized layer of atmosphere. Consider also, in conjunction with the` above, the phenomenon which occurs almost universally in practice, of

the rotation of the plane or" polarization, such as may be caused, for example,-by the earths magnetic eld. The resultant effect, -so far as concerns the present invention, is@the existence of a horizontal electric eld perpendicularY to the direction of vpropagation in the horizontal plane.

This component Ywill haveno eiect on the vertical conductors of a loop, which Will be, under thesey circumstances, still perpendicular to it.

However, this horizontal component will introduce an electromotiveforce into the horizontal perpendicular to the direction of arrival ofthe indicate the direction of incidence. In the practical operation of the loop to obtain a null position, the loop Will .be rotated about its vertical axis until the electromotive forces generated in ,the vertical sides are such as to compensate for the electromotive forces generatedV as above in the horizontal portions. This means that in the i null position `of the loop, the normal to the'plane lof the loop will no longer point toward the distant station, the variation fromr the direction of that station being a measure of the bearing error.

The two figures 'of 'the drawing disclose a type of apparatus which may be manipulated in such quantity..EHy-Eg/H, given above, is zero, and l normal tothevdirection of propagation.V

The apparatus employs an open antenna 1 in Y conjunction with a loop antenna 2 coplanar therewith, or at least occupying a plane parallel thereriparata component of electric 'eld in its direction of extension, and correspondingly, the loop antenna l gives a response, for a given frequency, numerimeasures the product of the output of the loop i Poynting vector expression EaHy-EyHm.

makes possible, by the measurement of two such and open antennae, there willresult an indication which, A.the antenna system is properly orientated,represents either term inthe above productsfcorresponding to two right-angularly related orientations of thexsystem about an axis 'the position of which corresponds to the Zidirection desired,'the ascertainment of the value of the Poynting vector -in such direction.,

x In one practical embodiment of the invention, and as disclosed in Figi, the open and loop antennae are rigidly joined together so as to occupy la common plane, Whichis made capable of rotation about a horizontalfaxisas above represented by reference numeral 3.' This horizontal axis in turn is capable of rotation about a vertical axis represented by reference numeralv 4, vso that theY plane of the antenn is capable of rotation simultaneously of'two right-angularly relates axes.

As in accordance with the procedure in using the labove apparatus for determining the 'direction of propagation by determining the direction of the Poynting vector, the above product of E and H is 'measured with the horizontal 'axis 3 fixed, for vertical and horizontal positions of the plane, as by rotation of the plane about the axis 3. These two sets of readings provide la' determinationV of the value of the quantity VEaHy-EyHe, aboved discusssed. Successive sets of such readings are taken for diierent positions Vof the axisv 3, by rotation about axis 4, until a null position for this quantity EHy-EyHa has algebraicV sum of the two readings corresponding to the two positions of the antenna systemabout the horizontal axis is equal toxthe corresponding Ytr'atalenergy vector for that particular-position of the system. With this positioning, the null.

value indicates that no energy ispropagated in the direction of the e axis. Accordingly, the direction of propagation is accurately determined as normal to this direction.

An alternative method, which4 may have some advantage over the one above described on account of the fact that'it is not a null method, is as follows: Readings similar to those described above are taken with the horizontal axis in a direction approximately 45 from the direction of propagation. The horizontal axis is` then rotated about the vertical axis to a position which givesan equal energy reading. The mean of these two directions is the true directionV of propagation.

In order to obtain the product of the output of the two antennae, it is convenient, although not always necessary, to step down the frequency to a value comparable with that of the currents to which a conventional-Wattmeter is adapted to rediifers from the frequency of the incoming Wave by, for example, lOGcycles. The incoming waves Vderived from the open antenna and loop anten- VAsa result of these This na 2 are combined with the wave from this oscillator in, respectively, detectors 6 and 7, the difference frequency, assumed to be of the order of 100 cycles, being selected by tuned circuits 8 and 9 respectively, and combined inthe multiplying device 10 after amplification, if desired', by ampliers 1l and 12.A f

APhase shifter 13, which may be interposed between the oscillator and either detector, but which is shown related to the detector 6, insures that the quantities to be eventually combined in device kl() have the requisite phase relation. This relationv should be such that'the product is a maximum when alknown local signal isbeirigr received from .a horizontal direction. 1n generalthe local transmitting source should be at a distance of several wave lengths from the receiver. Y

Itis obvious that theprinciple of the invention inheres equally as well asin the above described system in a systemin which the frequency step-down is dispensed with or in which other f types of multiplying devices than those partakly, and since, in the form disclosed, their circuits are such as are conventionally used in simple types of detectors, a'description of their various elements and associated structure is not necessary, `and the same thing is true of the open and loop antenn. It may be observed, however, that it is a function of the variable condenser v1li associated with the loop to tune the loop to the desired frequency, and that it is a function of the condenser l5r to perfect a symmetry between the loop .and combining means 7 so as ultimately to insure an electrostatic balance to ground, so as to avoid what is commonly lknown as the an-V tenna effect, that is, to avoid any tendency of the loop to function by virtue of its electrostatic rather than its electromagnetic exposure.

' Since the principles of the invention are applicabsle whether the vopen antenna'actually 0ccupies the plane of the loop or is parallel with said plane butclistant Vfrom it a small fraction of a wave length, it is desirable in claim drafting to employ a single expression including each of these concepts. Accordingly it should be understood that in the claims an expression of the fact that the open antenna is parallel with the plane of the loop or the like, should be taken to cover all degrees of separation including zero separation and therefore to cover a case where the open tion of arrivalof the waves.

2. The method of determining the direction of -arrival of a wave in a given plane comprising vmeasuring the product ofthe electric and magneticv fields about two right-angularly related axes inthat plane, similarly measuring such product with the directions of said fields interjchanged, repeating like sets oi measurement for various angulardisplacements of the system of `axes about avertical axis and determining'the angular displacements when the dierence of said products is a minimum.'l

3. The method of the preceding claim in which the given plane is the earths plane whereby the directions of the axes in said plane for the posi. tion of the system of axes corresponding to the minimum difference of said products correctly indicates the direction of propagation of the Wave in the earths plane.

4. The method of direction finding, usingan antenna systemcornprising an openand 4a loop antenna, which comprises measuring the product of the responses of said antenna to an incoming wave when the openV antennavand loop are both vertme'spondingly measuring such product when said antennae have the same relation to eachother as before, butwhen the loop has been rotated about an axis normal Vto the Yopen antenna until it occupies a' horizontal plane,

observing the diierence between the two prodi-o ucts, repeating such product measurements and observations for Various orientations of the antennasystem about the vertical axis, and determining the orientation of the system when the` diiferencel between the two products is a minimum. Y

5. The method defined in the preceding claim characterized by the additional step of steppingdown the frequency of the response to a sub'- stantially lower value before making the product measurements, said step-down being equal for all such product measurements.

6. In combination, an antenna system comprising a unitary combination of anopen and a A loop antenna, said open antenna being parallel withthe plane of said loop, means for orientatng such system about a horizontal and a Vertical axis independently, and means formeasuring the product of the responses of said antennae to aniincoming wave for any desired orientation of such system about either of its aXes. Y

7. The combination specified in the preceding claim comprising additionally', means between said antenna'and said measuring means for sub` stantially stepping-down the frequency of response of said antennae. Y

JOHN C. SCHELLENG. 

